1. Field of the Invention
The present invention relates to a rotation measuring mechanism to measure a rotation amount of a rotary member (typically a roller) by detecting a detection object rotating about a rotational axis of the rotary member in synchrony with the rotary member, and further relates to a sheet conveyance apparatus, original reading apparatus, and image forming apparatus using the rotation measuring mechanism.
2. Discussion of the Related Art
Rotation measuring mechanisms to measure a rotation amount of a rotary member such as a roller or a motor rotor have conventionally been used in various fields. For example, JP-3048685-B discloses an image forming apparatus using a rotation measuring mechanism to measure a rotation amount of a separation roller. The rotation measuring mechanism of this image forming apparatus includes a slit disc fixed to a rotation axis of the separation roller separating recording sheets fed out from a sheet feed cassette one by one and rotates together with the separation roller. The slit disc, as a detection target, is provided with a plurality of radial slits arranged at predetermined intervals in a rotation direction. A transmission photosensor provided in proximity to this slit disc has a light emitting element and a light receiving element, and directs light beams emitted from the light emitting element toward the slit disc. When the radiated light beams strike the surface of the slit disc, they are reflected, but when the slits in the slit disc are displaced to a position opposite the light emitting diode, the radiated light beams pass through the slit and are received by the light receiving element of the transmission photosensor. The transmission photosensor can detect the slit as a characteristic part of the slit disc, and the rotation amount of the separation roller can then be measured based on the number of slits detected by the transmission photosensor.
The image forming apparatus of JP-3048685-B is configured to detect slippage of a recording sheet on the surface of the separation roller based on the above measuring result. When slippage is detected, by increasing the pressing force of the separation roller against the recording sheets, the occurrence of slippage of the sheet is prevented and the feeding of multiple recording sheets simultaneously can be prevented.
In addition to the technology disclosed in JP-3048685-B, a technology is known in which a magnet as a detection target and a Hall sensor to detect magnetic force as a characteristic part of the magnet are used in combination, thereby measuring the rotation amount of the rotary member.
In recent years, with further diversification of equipment desired, it may be necessary to detect displacement in a direction perpendicular to the axis of the rotary member in addition to the rotation amount of the rotary member. For example, the image forming apparatus as disclosed in JP-3048685-B is designed to detect a thickness of the recording sheets, and therefore it may be required to detect displacement of the separation roller in the direction perpendicular to the axis. In such a case, providing another sensor to measure the roller displacement amount in addition to the sensor to measure the roller rotation amount increases the size of the apparatus, which is generally undesirable.